Imac spits out DVD

I can read the snow leopard DVD but Im trying to install photoshop and my 24" iMac ejects the DVD. Im assuming Photoshop is on a DVD and not a CD. There is no marking on the disk telling which it is and snow leopard says DVD so im trying to read the same format and one works and the other doesn't. Photoshop lightroom 3 says windows/mac os on the disk.

It doesn't make any difference if the media is a CD or DVD, if it's not being read and instead is ejected, then something is either wrong with the disk or your superdrive. Look the disk over for any scratches or marks. You might also try cleaning the superdrive. To clean the drive, buy a can of compressed air and blow a couple of bursts into the drive. Then try reading the disk again.

If the rollers can't suck the disc in fully, closing a microswitch, the drive times out and reverses the motor, ejecting the disc. Cleaning the dust away from the switch and rollers on the left of the drive slot (looking at it as if the computer's laid screen-down) can solve this. If not, it's new drive time unfortunately.